1. Field of the Invention
This invention relates to a laser diode (LD) module of optical communications system relying upon optical fibers.
This application claims the priority of Japanese Patent Application No. 344843/2000 filed on Nov. 13, 2000 which is incorporated herein by reference.
2. Description of Related Art
FIG. 26 shows an axially-symmetric metal-canned laser diode (LD) module which is one of the most prevalent laser diode modules at present. The predominant LD module has a circular metallic stem 80 having an erect pole 84 on the top, lead pines 90 projecting from the bottom of the stem, an LD chip 85 mounted in a vertical posture on the front wall of the pole 84, a monitoring photodiode (PD) 86 bonded on the stem beneath the LD 85, a cylindrical metallic cap 83 covering the LD 85 and the PD 86, a cylindrical metallic lens holder 81 having a lens 87 and being fitted upon the stem 80 and a conical metallic ferrule holder 82 with a ferrule 89 holding an optical fiber 88 and being soldered on the lens holder 81. The monitoring photodiode 86 is a xe2x80x9ctop incidencexe2x80x9d type PD which allows light to enter via a top opening around a ring p-electrode.
In the prior art metal-canned LD module, the lens 87, the LD 85 and the monitoring PD 86 align along an extension of the optical fiber 88. The metallic lens holder 81, the metallic stem 80 and the metallic ferrule holder 82 are all rotationally-symmetric around the axial line of the optical fiber 88. An IC 91 for driving the LD 85 is fitted upon a print circuit board 92 comprising an epoxy board, copper wiring patterns 93 printed on the epoxy board and many bores. The lead pins 90 are inserted in holes and soldered to wiring patterns of the print circuit board. The IC connected wiring patterns are joined to the electrodes of the LD driving IC 91. The LD driving IC 91 supplies pulse signal currents to the LD 85.
The metallic stem 80, the metallic lens holder 81 and the metallic ferrule holder 82 build a metallic package. The path of the light is in parallel to the axial line of the metal package. The monitoring PD 86, the LD 85, the lens 87 and the fiber 88 align along the axial line of the package. The metallic package repulses electromagnetic noise. The metal case has also high resistance against water and oxygen. Thus, the metal encapsulated LD module is an excellent device endowed with high reliability and long life time.
The vertically sustained LD 85 emits light in both upward and downward directions. The upward light from the LD 85 transmits signal light which is introduced by the lens 87 into the fiber 88. The downward light from the LD 85 is power-monitoring light which is sensed by the monitoring PD 86. The LD power is sent from the monitoring PD 86 to the LD driving IC 91. The LD driving IC 91 controls the LD via a feedback circuit for maintaining a constant output power in spite of a change of the surrounding temperature or degradation due to aging. The LD driving IC 91 has a function of adjusting the LD power in addition to the role of making signal pulse currents.
The metallic package has a three dimensional structure having the lead pins protruding from the bottom and the fiber at the top. The light progresses in the direction orthogonal to the stem. The signal light from the LD propagates in the space, the lens and the space in series to the fiber. The space propagation disperses the light. The lens is indispensable for converging the dispersed light. The discrete parts should be allocated to the optimum positions which allow the LD to inject the maximum power into the fiber. The optimum positions are determined by measuring the output power at another end of the fiber with displacing the lens holder, the ferrule holder in the x-, y- and z-directions with respect to the stem. The process of allotting the parts to the most suitable spots is called xe2x80x9calignmentxe2x80x9d.
The print circuit board 92 sustains the LD driving IC 91. The LD 85 is stored in the metallic case. The LD 85 and the LD driving IC 91 are separated. Drawbacks accompany the prevalent LD module containing the metal cased LD module and the circuit board loaded LD driving IC. The current LD module is bulky, since it contains the print circuit board for maintaining the LD driving IC. The module requires many discrete elements, which raises the parts cost and the assembling cost. The existence of the discrete elements impedes miniaturization of the modules. High cost and big size of the modules inhibit the optical communications networks from prevailing on a large scale. Further development of the optical communications requires miniaturization and cost reduction of the necessary devices.
Planar lightguide circuit (PLC) type optoelectronic modules have been proposed for the sake of propelling miniaturization. Various types of PLC modules have been suggested till now. The proposed PLCs are all still suffering from some difficulties. There are no PLC devices satisfying all the basic requests yet.
{circle around (1)} German Patent DE 43 13 492 C1, xe2x80x9cAnordnung zur Ankopplung eines optoelektronischen Empfangselementes an ein optoelektronisches Sendeelementxe2x80x9d, proposed an LD module which employs a bottom incidence type PD as a monitoring photodiode (PD). The newly proposed PLC LD module has a silicon substrate with a front V-groove and a rear V-groove dug along the central line of the substrate, an LD chip mounted upon a plateau between the front and the rear V-grooves, a cylindrical lens mounted on the substrate before the front V-groove, an optical fiber directly joined to the lens and a monitoring PD over the rear V-groove on the substrate. On the silicon substrate, the fiber, the cylindrical lens, the front V-groove, the LD, the rear V-groove and the monitoring PD align in series from the front to the back. The LD chip emits signal light in both directions from the front end and the back end. The front light is converged by the lens and is introduced into the fiber. The rear light emanating from the LD is power-monitoring light which is reflected by banks or an end mirror plane of the rear V-groove and is guided into the PD via the bottom. The bottom incidence type PD requires a change of light path. The rear V-groove having the banks and the end mirror respond the request. The beam line of the LD is parallel to the central line of the silicon substrate. Unlike the previous LD module as shown in FIG. 26, the PLC type module has two dimensional character, which simplifies the structure of the package. The most significant advantage is to eliminate the time-consuming positive alignment. The position of the fiber is definitely positioned by the V-groove and the positions of the LD and PD are exactly determined by marks attached on the substrate. The simplified structure is favorable for miniaturization. The rear V-groove enables the bottom incidence PD to catch nearly half of the rear light emanating from the LD in the rear direction. The prior art had no LD driving IC on the substrate. Like the previous art of FIG. 26, the LD driving IC would be mounted upon another print circuit board and would be joined to the LD by wiring patterns and bonding wires.
{circle around (2)} Y Akahori, T. Ohyama, M. Yanagisawa, Y Yamada, H. Tsunetsugu, Y Akatsu, M. Togashi, S. Mino and Y. Shibata, xe2x80x9cA HYBRID HIGH-SPEED SILICA-BASED PLANAR LIGHTWAVE CIRCUIT PLATFORM INTEGRATING A LASER DIODE AND A DRIVER ICxe2x80x9d, ECOC 97, Sep. 22-25, 1997, Conference Publication No. 448, IEE, 1997, p 359-362 proposed an improvement of loading LDs and a driving IC on a silicon bench. FIG. 1 of {circle around (2)} is shown as FIG. 27 here. An LD 96 is one of an LD array which includes a plurality of LD chips. The LD array is driven by a single driving IC 97. The transmitting distance is very short of several meters to several tens of meters. The signal repetition rate is 9 Gbps. The LD driving IC 97 is positioned just behind the LD 96 for ensuring the high signal transmission rate of 9 Gbps.
Since the LD driving IC occupies the rear terrace at the back of the LD, the module can allocate no monitoring PDs on the substrate. The omission of the monitoring PDs causes instability of the LD for a change of temperature or incompetence for treating with the degeneracy by aging. The LD array module should be used in tightly air-conditioned circumstances maintaining a constant surrounding temperature. This is only a single prior art having the driving IC on the same substrate as the LD.
High speed transmission forces the LD driving IC to be close to the LD. However, the monitoring PD is also important for regulating the output power of the LD.
The inventors of the present invention found no prior art module having the monitoring PD and the driving IC on the same substrate as the LD. The driving IC is as important for the LD as the monitoring PD.
Significant requests for LD/PD modules are miniaturization, low-cost and high signal frequency. The present invention pays attention in particular, to an LD module for heightening the transmission signal frequency. The current signal repetition rates are 156 Mbps or 622 Mbps. But near future requires 1.25 Gbps or 2.5 Gbps for a signal repetition rate. Perhaps far higher repetition rates of 5 Gbps and 10 Gbps will be required in not so far future.
The high speed signal rate causes new problems on both transmitting devices (LD modules) and receiving devices (PD modules). In the LD module, the LD and the IC are connected by thin wires and metallized patterns printed on the substrate. The LD driving current is large and the LD input impedance is low. The current signal repetition rates of 156 Mbps and 622 Mbps cause no problem owing to the large LD current and the low LD impedance. Signal repetition rates higher than 1 Gbps will incur a problem of signal distortion owing to the inductance of the wiring (wires and patterns) connecting the LD and the LD driving IC. The self-inductance of the LD/IC wiring is denoted by xe2x80x9cLxe2x80x9d. xe2x80x9cLxe2x80x9d is in proportion to the length of the medium. The wiring has a complex impedance of xe2x80x9cjxcfx89Lxe2x80x9d, where xcfx89 is an angular frequency (xcfx89=2xcfx80f) and j is a unit of a imaginary number. The impedance is in proportion to f.
When the signal speed is low, the impedance is negligible. However, the wiring impedance jxcfx89L is innegligible for high repetition rate signals, because jxcfx89L increases in proportion to the signal frequency f. The large wiring impedance induces a decrease of the pulse voltage applied upon the LD, incurs a signal delay of the LD and causes distortion of the LD signal pulses. The distortion and the delay prohibit the LD from generating regular pulses at a high repetition rate.
The prevalent prior art of FIG. 26 connects the LD 85 in the metallic case (80, 81, 82) via the pole 84, the wire, the stem 80, the leadpins 90 and patterns 93 to the LD driving IC 91 bonded upon another print circuit board 92. The inductance L increases in proportion to the length. The LD/IC wiring including the pole, the wire, the stem, the leadpins and the print patterns has a large self inductance xe2x80x9cLxe2x80x9d. For example, a 1 mm long wire (30 xcexcmxcfx86) has 1 nH (nanohenry=10xe2x88x929 henry) of self inductance. The prior art of FIG. 26 is incompetent for high repetition rate communications of 2.5 Gbps or 5 Gbps due to the large impedance of the wiring between the LD and the IC.
The prior art of FIG. 27 proposes access of the IC 97 to the LD 96. The LD is interposed between the waveguide and the IC. There is no room for a PD. Geometrical conditions prohibit the FIG. 27 device from attaching a monitoring PD. The spatial restriction forces the FIG. 27 module to eliminate a PD for monitoring the LD power. FIG. 27 lacks a monitoring PD. The IC is not aware of degeneration or aging of the LD. Exclusion of the monitoring PD forces the module to give up the controllability of the LD. Loss of the LD controllability invites instability and malfunction. The module forfeits reliability by high probability of malfunction.
The inventors of the present invention considered over the arrangement of the LD, the LD driving IC and the monitoring PD. The PD succeeds the LD in prior modules. The PD is positioned at a spot close to the LD for receiving the LD light as much as possible. The skilled believe in the access of the PD to the LD as a matter of course. Long consideration taught the inventors that the monitoring PD was not necessarily close to the LD but could be distanced from the LD, so long as the PD could catch the light emitted from the rear of the LD. The inventors were aware that geometric access was not a requisite condition for the reception of the LD light. The inventors found the fact that the access of the driving IC to the LD was more important than the access of the monitoring PD to the LD. The inventors tried to mount both the IC and the PD near the LD and succeeded in giving both the PD and the IC access to the LD.
One purpose of the present invention is to provide an LD module succeeding in the access of the IC to the LD without omission of the monitoring PD. Another purpose of the present invention is to provide an LD module with a far smaller inductance of wiring between the LD and the IC than prior ones. A further purpose is to provide to an LD module suitable for high repetition rate transmission. A further purpose of the present invention is to provide an LD module ensuring controllability of the LD power even for high repetition rate transmission. Another purpose of the present invention is to provide an LD module maintaining reliability for high repetition rate transmission.
The present invention positions the IC next to the LD and the PD following the IC and detours the LD rear light below the IC to the PD. Namely this invention aligns the LD, the IC and the PD in this order. The LD rear beams detour below the IC and attains to the PD. An outputting medium (optical fiber or waveguide) precedes the LD for sending the LD forward light to other nodes (or stations) in an actual module. The outputting medium, the LD, the IC, the detouring device and the PD are preferably covered with a transparent resin.
There is no prior art which leads the LD light to the PD by detouring the LD light around the IC. Nobody has suggested such a detour of light around an obstacle. Without an idea of detouring, the PD could not be mounted on the module. The detouring enables the LD module having the driving IC to carry a monitoring PD.
The novel LD/IC/PD structure of the present invention brings about the access of the IC to the LD. The IC/LD access shortens the wiring path between the LD and the IC. Shortening of the wiring decreases the inductance xe2x80x9cLxe2x80x9d and impedance xe2x80x9cjxcfx89Lxe2x80x9d. Reduction of jxcfx89L of the LD/IC wiring suppresses the signal delay and the signal distortion.
The access of the IC to the LD separates the PD from the LD. The IC is an obstacle for the coupling of the PD and the LD. The LD rear beams are detoured for avoiding the IC and are guided to the PD positioned at the backmost in the series of the LD/IC/PD.
The advantages of the present invention are described. The present invention proposes a novel LD module having a series of an LD, an IC and a PD on a substrate. The IC/LD access shortens the wires, and the patterns, which reduces signal distortion and signal delay.
The LD device has an advantage which is suitable for high speed transmission of 5 Gbps to 10 Gbps in addition to the low-cost and the miniaturization.